JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. B6, PAGES 12,529-12,543,JUNE 10, 1998
Compressional velocity structure and anisotropy
in the uppermost mantle beneath Italy
and surrounding regions
GiulianaMele,1 AntonioRovelli,
• DoganSeber,
2
ThomasM. Hearn,
3 andMuawiaBarazangi
2
Abstract. Travel timesof about39,000Pn arrivalsrecordedfromregionalearthquakes
by the
ItalianTelemeteredSeismicNetworkandby stations
of nearbycountries
areinvertedto image
lateralvariationsof seismicvelocityandanisotropy
at subcrustal
depthin Italy andsurrounding
regions.This methodallowssimultaneous
imagingof variationsof Pn velocityandanisotropy,as
well as crustalthicknessvariations.The Po plain, the Adriatic Sea,andthe Ionian Seahave
normalto high Pn velocities.In contrast,lower velocities(7.9-8.0 kin/s) are imagedin Italy
beneaththewesternAlps,thenorthernApennines,
andeasternSicilyandnearbyCalabria,aswell
as in northern Albania and beneath the Pannonian basin. Low Pn velocities beneath the northern
Apenninescorrelatewith present-day
extensionandmay haveresultedfrom thermalanomaliesin
the uppermostmantlepossiblydueto delaminationprocesses.
Low velocitiesare consistentwith
thehigh-attenuation
zoneinferredin theuppermost
mantlebeneaththeinternalApennineunitsand
theTyrrhenianmarginof thepeninsulaby Mele et al. [1996, 1997].On the contrary,low
velocitiesbeneaththe westernAlps may be an apparenteffectdueto the abruptthickeningof the
crustalroots.Pn velocityis anisotropicin the studyareawith a maximumamplitudeof + 0.2
km/s.The largestanisotropicvelocityanomaliesareobservedalongthemajorarc structures
of
Italy, i.e., the northernApenninesandthe CalabrianArc, indicatingthatthesefeaturesare
controlledby uppermostmantleprocesses.
The anisotropyanomalyalongthe CalabrianArc
extendsas far as Albania but endsabruptlynorthof thisarea,suggesting
thata lithospheric
discontinuityis presentalongthe northernAlbanianborder.
1. Introduction
The inversionof Pn travel times providesinformationon
the velocityand anisotropystructures
at the topmostpart of
the mantle where the P n wave propagates. Previous
tomographystudiesof the uppermantlehave usedisotropic
techniques,
and only very recently,researchers
are trying to
solvebothfor velocityand anisotropy[seeHearn, 1996].The
presenceof seismicanisotropywithin the uppermostmantle
and orthopyroxenecrystals.These minerals,which are the
mostabundantand anisotropic
in uppermantlerocks,havea
strongtendencyto preferentiallyorient duringdeformation,
and even a slight statistical orientation of their
crystallographic
axes can producesignificantdifferencesin
seismic velocities that influence seismic travel times. To
account for this, we have included the lateral variations of
azimuthal anisotropy in our inversion of the Pn data. The
presentstudy,togetherwith the studyof Hearnand Wu [ 1995],
is now well establishedfrom numerousseismicmeasurements, is the first attemptof imagingthe velocitystructurein the
xenolith samples,and laboratoryexperiments[e.g., Hess, uppermostmantle of Italy by taking into accountpossible
1964;Backus,1965;Raitt et al., 1971;Fuchs,1977;Barnford anisotropyeffects.$KS splittingmeasurements
carriedout at
et al., 1979; Beghoul and Barazangi, 1990; Christensen, eight stationsof a temporaryarray crossingthe northern
1977;Karato, 1989;Nicolasand Christensen,
1987;Savage Apennine chain are available in the study area [Margheriti et
and Silver, 1993; McNamara et al., 1994; Silver, 1996]. al., 1996].
Thesestudieshaveshownthatseismicanisotropy
represents
a
Italy extends into the central Mediterranean area and is
powerful tool to investigatethe strain field in the upper
characterized
by compressional
orogensto the north(Alps)
mantle becauseit revealsthe preferredorientationof olivine
and along the peninsula (Apennines) (Figure la). These
mountainbelts formedsincethe Cretaceous-Paleogene
in the
frameworkof the convergence
betweenthe major African and
llstituto
Nazionale
diGeofisica,
Rome,
Italy.
2Institute
for the Studyof the Continents
andDepartment
of
GeologicalSciences,
CornellUniversity,Ithaca,New York.
Eurasianplates[e.g., Hsii, 1971; McKenzie, 1972;Laubscher,
1975; Dewey et al., 1989]. In this period, lithosphere
subduction and continental collision dominated the evolution
3Department
ofPhysics,
NewMexico
State
University,
LasCruces. of the Mediterranean area, whose present setting is
characterized
by large regionsof extensionsurrounded
by
Copyright1998by theAmericanGeophysical
Union.
Paper number 98JB00596.
0148-0227/98/98JB-00596509.00
arcuatebelts of compression[e.g., Hsii, 1977; Horvath and
Berckhemer, 1982]. Crustal thicknessin Italy varies from
about 30 to 40 km within the continentallithosphere
12,529
12,530
MELE• AL.:PnVELOCITY
ANDANISOTROPY
IN ITALY
Adriatic
Sea
Tyrrhenian
2
3
4
Figure1. Map of Italy andsurrounding
regionsshowing
the mainstructural
andphysiographic
features.(a)
Legenddefines1, mainthrusts;
2, normalandverticalfaults;3, volcanics;
4, oceanicbasins.S-P on mapis
the Scutari-Pec
shearzone.Activecompression
in the northernApennines
is indicatedwith barbedthrusts.(b)
MapoftheMohodepth
(inkm)afterNicolich
andDalPiaz[1981].
(c)Heatflowmeasurements
(inmW/m
2)
after Loddo and Mongelli [1978].
underlyingthe peninsulaandthe AdriaticSea,as well as the
Corsica-Sardiniaarea. Thinned crust is presentbeneaththe
Tyrrhenian and Ionian Seas, while deep crustal roots
tectonicspresentlydominatethe westernside of the Italian
peninsula, from Tuscany to Calabria, where extensional
deformation
has
been
active
since
the
Pliocene.
The
characterizethe westernAlps where the Moho discontinuity extensionaltectonicsoverprintedthe compressionalfeatures
deepens
abruptlyfrom20 to 55 km (Figurelb) [Geiss,1987; of the internal Apennine units, as can be observedin the
northern Apennines. Extension is active along the whole
Nicolich and Dal Piaz, 1990].
During the late Oligocene-early Miocene, a length of the chain and coexists with active compression
counterclockwise
rotationbroughtthe Corsica-Sardinia
block along the most external thrustsof northernApennines[e.g.,
in its presentposition(Figure la) [e.g., Channellet al., Elter et al., 1975; Lavecchia, 1988, and referencestherein].
¾olcanicsin northernTyrrhenianand Tuscanyis acidic in
1979; Montigny et al., 1981]. Westward subductionof
the
intrusive bodies of Mio-Pliocene age, while south of
lithosphere
beneaththe Corsica-Sardinia
marginproduced
the
first compressional
deformation
of the Apenninesanda phase Tuscany it is composedby highly K-alkaline seriesof late
Plio-Quaternaryage, relatedto the intenserifting that affected
of calk-alkaline volcanism in western Sardinia [e.g.,
the west coastsof Italy [e.g., Barberi et al., 1973; Beccaluva
upperTortonian,extensionstartedin the Tyrrhenianarea, et al., 1989]. Within the abyssalplain of the TyrrhenianSea
while coeval compression
continuedto affect the Apennine several seamountsof Pliocene tholeiitic basalts are present.
Beccaluvaet al., 1989; Patacca and Scandone,1989]. In the
belt duringa counterclockwise
rotationaboutits northernend
[e.g., Civettaet al., 1978; Pataccaet al., 1990]. Sincethe
Messinianthe Apennineshave beendividedinto two major
segments
mergingin centralItaly [Locardi, 1988]. Each
segmentwas affectedby differentamountof shortening
and
rotation[e.g.,Malinvernoand Ryan,1986;Laubscher,1988;
To the south, andesitic series characterize the Quaternary
volcanism of the Eolian Islands, while continental basalts
outcrop in eastern Sicily. Volcanic activity still occurs in
these areas.
Heat flow measurementsin Italy show a regional pattern
characterized by increasing values from the Adriatic and
to 100mW/m
2ontheTyrrhenian
Pataccaet al.. 1990]. Collapseby normalfaultingand graben Ioniansideof thepeninsula
MELE ET AL.' Pn VELOCITY AND ANISOTROPY IN ITALY
side (Figure l c) [Loddo and Mongelli, 1978]. Values as high
2.
12,531
Data
as200to 400mW/m2 arefoundin Tuscany
andin thesouthern
TyrrhenianSea [Cataldi et al., 1995].
Abundant crustal seismicity occurs along the mountain
belts of Italy and surrounding regions. Focal mechanism
solutions indicate a NE-SW direction of extension along the
Apennines, while predominant NW-SE thrust faulting on
shallow dipping planescharacterizesthe Dinaric and Hellenic
belts [e.g., Anderson and Jackson, 1987]. Intermediate-depth
seismicity is found within the upper 90 km beneath the
northernApennines[Selvaggiand Amato, 1992]. Earthquakes
as deep as 600 km define a NW dipping Benioff zone beneath
the southernTyrrhenian Sea [e.g., Ritsema, 1972; Gasparini
et al., 1982; Giardini and Velona, 1991' Istituto Nazionale di
Geofisica (ING), 1997].
Previous studies of seismic isotropic tomography in the
Mediterranean area have imaged the velocity structureof the
upper mantle beneath Italy. These studies indicate that
remarkable lateral heterogeneities exist at various depths'
low-velocity anomaliesare found within the upper 200 km of
depth in the northern half of the peninsula [Babuska and
Plomerovd, 1990; Spakman, 1991' Spakman et al., 1993;
Piromallo and Morelli, 1997], while high velocities appear
below 200 km of depth and are interpreted as remnant of
subductedlithosphere[Wortel and Spakman,1992]. Beneath
the northernApennine arc high velocities have been imaged
in the depthrangeof 80-140 km by Cimini and Amato [1993]
and Cimini and De Gori [1997]. Subducted lithosphere is
imaged at depth within the Tyrrhenian deep seismic zone
[e.g., Wortel and Spakman'1992; Selvaggi and Chiarabba,
1995' Piromallo and Morelli, 1997].
Several models of the geodynamicevolution of Italy in the
context of the Mediterranean area have been proposed
[Boccalettiand Guazzone,1974; Malinverno and Ryan, 1986;
Lavecchia,
1988' Locardi,
1988; Patacca et al., 1990'
Doglioni, 1991' Mantovani et al., 1996, among many
others]. An interestingand debatedtopic is representedby the
...... ;..... •,,•;•,.• ,,c the northern Apennines a'-d •"' its
relationship with the lithosphericstructureand evolution of
the southern Apennines/Calabrian Arc. Geophysical and
geological studies mentioned above in this section outline
significantdifferencesbetween these two areas.
-6
0
In this study we use Pn travel times recordedby the stations
of the Italian Telemetered Seismic Network (ITSN), which is
run by the Istituto Nazionale di Geofisica (ING), and by
stations of surrounding countries. Several thousand
earthquakesof shallow depth are yearly recordedin the study
area, mainly occurring along the peri-Adriatic mountain belts.
From 1988 to 1994 a total of 59,706 Pn arrival times from
6049 shallow events (source depth < 30 km) have been
recorded by 177 stations in Italy and nearby regions. The
uppermost mantle phase Pn is the first arrival recorded at
regional distances from shallow earthquakes. First arrival
times labeled as "Pn" are taken from the ING bulletin starting
from about 200 km of epicentral distance. The maximum
epicentraldistanceallowed in our data set is 1000 km. Travel
times were obtained using origin times and locations from the
ING bulletin and then corrected for Earth's sphericity and for
the station site topography assuming an average seismic
velocity of 5.5 km/s in the surfacelayer. Travel time residuals
largerthan• s wereeliminated
from the dataset.These
residuals are significantly larger than the data scatter and
represent clear outliers. Events recorded by less than 10
stations,and stationsthat recordedless than 10 events during
the studyperiod were also eliminatedfrom the data. A linear fit
to the final data set of Pn travel times yields the average Pn
velocity in the region. Residuals were recalculated with
respect to this best fit and a maximum residual of 6 s was
allowed in the data set (Figure 2).
As a result, a total of 38,868 Pn travel time residuals from
2745 events recorded by 167 stations met the selection
criteria and were used for the tomographyanalysis. A total of
30,561
travel times, i.e., about 80% of the data set, are
recordedat epicentraldistancesof 2ø to 5ø, 7170 travel times
are recorded between 5 ø and 8 ø, 1137 travel times are recorded
between 8ø and 9ø. The resulting Pn ray paths provide very
good coveragethroughoutthe study area (Figure 3).
3. Tomography Method
The Pn travel time residualsshown in Figure 2 have been
inverted to model lateral variations in the uppermost mantle
I
I
I
I
200
400
600
800
'
I
1000
'
1200
Distance (km)
Figure 2. Travel time residualsrelativeto a linearfit. ApparentPn velocityis 8.2 km/s. The intercepttime is 7.3 s.
12,532
MELEET AL.:PnVELOCITYANDANISOTROPY
IN ITALY
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10øE
50øN
-
20øE
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Figure 3. Map showingthe 38,868 Pn ray pathsusedfor the tomography
analysis.The ray coverageis
very denseover the Italian peninsulaand the Adriatic Sea.
the unknown slowness and anisotropic coefficients. A
measure of the relative trade-off between the velocity
variations and the anisotropyvariations is obtained by using
the root-mean-square(rms) value of the magnitude of the
along the propagationpath: (1) from the hypocenterto the velocity perturbations and the rms of the anisotropy
mantle; (2) through the uppermostmantle; and (3) from the magnitude perturbations for each inversion. Two separate
mantle to the receiving station. The validity of the ray path inversions were run to do this: a test model with only
modeling as a refraction path is supported by the linear
checkerboardvelocity anomaliesand anothermodel with only
behavior of the Pn travel times versus distance (Figure 2).
checkerboardanisotropyanomalies.The final pair of damping
Terms I and 3 are parameterizedby event and station static constants was chosen so that the ratio of the rms velocity
delays, respectively, while the variation of seismic velocity perturbations to the rms anisotropy perturbations and its
within the uppermostmantle is parameterizedby subdividing inverse were equal for the two models (see Hearn [1996] for
the uppermostmantlein a two-dimensionalgrid of squarecells details).
The solution variances are estimated using the bootstrap
of appropriatesize. With our data set we used one-quarter
degree squaredcells. The three legs of the Pn travel time are method,which repeatedlyinverts subsetsof the actual data set
describedwith a time-termequationof Hearn [ 1996] as
[Efron, 1979; Koch, 1992]. The resolutionof the inversion is
best investigatedby checkerboardtest models.
velocity and anisotropy following Hearn [1996]. For the
epicentral distances used in this study the Pn phases are
modeled as refracted rays bottoming below the Moho
discontinuity. Their travel times separate into three parts
tO=ai+bj +5'.dt..ilc(slc
+A/ccos2½
+B/csin2½)
(1)
where
ai andbjarethestatic
delays
forevent
i andstation
j,
4. Results
respectively,
d{ik isthedistance
theraytravels
inthekthcell
The slope of the linear fit to the selectedPn travel times
and is summedover all cells traversed,skis the slowness
(inversevelocity)perturbation
withincell k, At•andB•are the indicatesan averageuppermostmantle velocity of 8.2 krn/s in
anisotropycoefficients,and {bis the back-azimuthangle.
A major simplification for Pn wave anisotropyis made by
assuming that velocities in a plane are described by a 2½
azimuthal variation. The magnitude of the anisotropy is
definedas (A•2+B•2)t/2,andthe fastdirection
of P n
the studyarea. A mean crustalthicknessof 30 km is obtained
from the intercept time, assumingan averagecrustal P wave
velocityof 6.3 krn/sin thecrustanda meansource
depthof 10
km.
Lateral variations of P n velocity are imaged as
perturbations from the average velocity of 8.2 km/s.
of equationsis solved using a preconditionedversion of the Similarly, station delays represent variations in crustal
LSQR conjugategradientleast squaresalgorithm [Paige and thicknessand velocity relative to a 30 km crust of 6.3 km/s.
Saunders, 1982]. The algorithm is run well past convergence Event delays are not interpreted because they are strongly
to 100 iterations.
affected by the uncertainty in determining the depth and the
A set of Laplacian damping equations regularize the origin time of the earthquakes.
solution,and two dampingconstantsare separatelyappliedto
The trade-off between Pn velocity and anisotropy is
propagation
is givenby 1/2 arctan(B•!Ak).
The resultingset
MELE ET AL.: Pn VELOCITY AND ANISOTROPY IN ITALY
10ø E
o
12,533
20 ø
50 ø
50 ø N
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40 ø
40 ø
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'rrheL•Sea)
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-0.15
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10o
.
,
•
20ø
Plate 1. Uppermost
mantlevelocityin the studyarea.Normal-to-high
Pn velocities(8.1-8.4km/s)are
foundbeneaththe Po plainandthe AdriaticandIonianSeas.Lowervelocities
areimagedbeneathwestern
Alps,Apennines,
eastern
Sicily,northern
Albania,andthePannonian
basin.The velocitystructure
is not
well definedin the TyrrhenianSeabecauseof the sparseray coverage.
investigatedby comparingsolutionsobtainedusing different
combinationsof damping parametersboth for velocity and
anisotropy.In general,the main featuresin the velocity and
anisotropyfields are quite stableeventhoughthe extentof the
velocity anomalies and the amount of anisotropymay vary
among the different solutions.The dampingconstantsalso
variations shown by Mochizuki [1997]. However, variations
in both the velocity and anisotropyof the uppermostmantle
are now well acceptedand should be accountedfor in travel
time inversion. We cannot exclude anisotropy from our
inversion since previous studiesshow that there is anisotropy
in the upper mantle beneath Italy [Margheriti et al., 1996].
control the trade-off between low errors (high damping) and Despite the trade-off between velocity and anisotropy,Hearn
small resolutionwidth (low damping). The best solutionsfor
[1996] showed that both could be resolved for regions with
velocity and anisotropywill result from a pair of damping good ray path coverage.
constants that makes the level of noise and the resolution
The Pn velocity is observedto vary in the study area from
width acceptable.In this study,the bestsolutionsare obtained about 7.9 to 8.4 km/s (Plate 1), showing a normal-to-high
usingdampingconstantsof 500 for the velocityand 1000 for velocity zone that extends from northern Italy through the Po
the anisotropy.A hundredbootstrapinversioniterationswere plain and the Adriatic basin down to the Ionian basin. In
more than enough to get a stable estimate of the standard contrast, low-velocity anomalies are imaged beneath the
deviationsfor velocity and anisotropyin each cell and for the western Alps, the northern Apennines, easternmostSicily and
stationdelays.The ray coverageover the studyareaallowedto Calabria, northern Albania, and the Pannonian basin. The
resolve velocity variations in 3893 cells out of a total of velocity beneath the Tyrrhenian Sea is not well determineddue
6400.
to the poor coverage of the ray paths. The error for the
The inclusion of anisotropyin the solution makes only a estimated velocity perturbations is small over most of the
small improvementin the rms residualsdue to the intrinsic study area (Figure 4); largest errors are found where the ray
trade-off between velocity variations and anisotropy coverageis less denseand at the edgesof the image.
12,534
MELE ET AL.: Pn VELOCITYAND ANISOTROPYIN ITALY
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10OE
20øE
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I Velocity
Error(k•s>
m
o'
iiii
10'E
20'E
Figure 4. Average standarddeviation of velocity.
Azimuthal variationsof Pn velocity reacha maximumvalue
of + 0.2 kin/s, corresponding
to about5% of anisotropy:the
fastest direction of velocity follows two major arc-shaped
patterns roughly parallel to the northern Apennine and the
Calabrian arcs (Figure 5a). Consistent directions of
anisotropy extend through the Calabrian/Ionian area toward
northernAlbania,wheretheybecomerandom.In Figure5b are
shownthe errors in the directionand amountof anisotropy.
The largesterrorsin the estimateddirectionsof anisotropyare
presentwhere little or no anisotropyis found.
The Pn waves are modeled in this study as traveling
subhorizontallyjust below the Moho discontinuity.While a
correction for the Earth's sphericity is made to the travel
times, no uppermost mantle velocity gradient is assumed.
With a significant velocity gradient, long Pn ray pathswill
deviate from a simple head wave assumptionand dive into the
mantle lid, sampling higher velocities than the shorter ray
paths. However, there is no evidencefor any curvaturein the
travel times plotted in Figure 2 that would indicate such a
mantlegradient.Furthermore,about80% of the ray pathsused
in this study are short and range between 200 and 600 km of
length. Only a small percentage(about 20%) of the rays are
longer than 600 km, and all are shorterthan 1000 km.
The stationdelayscalculatedare shownin Figure 6a. They
are primarily influencedby the crustalthicknessand velocity
but are also sensitiveto systematictiming and picking errors.
Although thickness and velocity in the crust cannot be
separatelydeterminedfrom equation (1), variationsof crustal
thicknessare consideredto primarily affect the stationdelays;
a 1-s delay correspondsto about 10 km of variation of the
Moho depth[seeHearn and Ni, 1994]. In the studyarea,early
arrival times are found at most of the stationslying aroundthe
Tyrrhenian and Ionian Seas;this is consistentwith the crustal
thinning that characterizesthesebasin. The stationdelaysare
low within the peninsula, indicating a rather regular Moho
topography. Late station delays (1.0 to 1.5 s) are found in
Albania, as expectedfor the thick crust inferred in this area
[Geiss, 1987]. Figure 6b showsthe averagestandarddeviation
for the station delay estimates.The largest deviations(0.3 s)
affect stationsrecordinga low numberof events.
Checkerboardtest models were performed on a synthetic
data set computed using the raypaths of our data set. The
synthetic data were then inverted as the real data. This test
showsthat 2ø by 2ø sized anomaliesof 0.3 krn/s are resolvable
over most of the tomographyimage (Figure 7a). Distortions
are present in the Tyrrhenian Sea becauseof the sparseray
coverage.The stationdelays calculatedfrom the checkerboard
test (Figure 7b) indicatethat the trade-off betweenthe velocity
image and the computed station delays is not significant,
except at stationslocated on the northernedge of the image.
The trade-off between crustal delays and mantle velocity
anomaliesis more influencedby-the azimuthalray coverageat
each stationthan by a high numberof recordingsper station,
or ray pathsper cells.
5. Discussion
Seismically, the mantle lid (i.e., the mantle part of the
lithosphere) is a high-velocity, high-Q region at the top of
the mantle. This lid generally overlies the asthenosphere,a
low-velocity, 1ow-Q zone in the upper mantle. Seismic
velocities in the upper mantle are primarily controlled by
thermal anomaliesrather than by compositionaland pressure
effects. High-temperaturedislocation-relaxationand partial
melting are processes that can drastically lower mantle
velocity [e.g., Andersonand Spetzler, 1970; Chung, 1977;
Black and Braile, 1982].
High Pn velocitiesfound in the presentstudy(Plate 1) and
the high Q values found by Mele et al. [1996, 1997]
consistently characterize the uppermost mantle beneath the
Po plain, the Adriatic Sea and the northern Ionian Sea, as
expected in tectonically stable regions [see Molnar and
Oliver,
1969; Black and Braile, 1982; Hearn et al., 1991;
Hearn and Ni, 1994]. The Po plain and the Adriatic Sea overlie
MELEET AL.:PnVELOCITYAND ANISOTROPYIN ITALY
12,535
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40 ø
.......................
.......................
IIIIIIIll
IIIIIIIIII
////////111•
,
........................
*
/I//////11111
•/////////11111•
....................
/•//11111 1,
IIIIIIII
..............
//11111111111111
........
ß
0.0
+0.05
./////1•,.,,,,,,,,•!
/
+0.10
+0.15
+0.20
......
•
..
Pn anisotropy (km/s)
0o
10 ø
20 ø
Figure 5a. Pn anisotropy
in the studyarea.The fastdirectionof Pn velocityis drawn,andthebar lengthis
proportional
to the amountof anisotropy
in thatdirection.The insetat the top showsthe fastpolarization
directions
inferredfrom SKS splittingmeasurements
(lineswith arrowheads)
redrawnafterMargheritiet al.
[1996]; their lengthsare proportionalto the averagedelay time. Thick arrowsindicatethe directionof
compressionin the northernApennines.
velocitiesas high as 8.2-8.3 km/s. Such a sharptransitionis
consistent with the attenuation pattern of the uppermost
mantle shearphaseSn [Mele et al., 1997]. Slow Pn correlates
with attenuatedSn phases.Exampleseismograms
showingthe
propagationcharacteristics
of the regional Sn phasein the
studyarea are shownin Figure 8.
Lateral variations of Pn velocity characterizethe whole
Apennine chain: in the northern part the low-velocity
anomalyis muchmorepronounced,
while in the southernpart
low velocitiesare distributedalong a narrowbelt beneaththe
Ionian Seas.
Calabria/Sicily region. A discontinuityin the velocity field
A sharp lateral variation in the uppermost mantle appearsbetweenthesetwo sectorsof the chain (Plate 1). A
velocities follows the external front of northern Apennines similarpatternis observedin the anisotropy
map (Figure5a),
and the southernborder of the Po plain. Beneaththe northern where two main anisotropic fields single out beneath the
Apennines and their internal areas toward the Tyrrhenian northern and the southernApennines.
Evidenceof low-velocity, low-Q material in the uppermost
margin of Italy, Pn velocities as low as 7.9 krn/s are found
(Plate 1). In contrast,the Po plain and the Adriatic Sea have mantle beneaththe northernApenninesis in agreementwith
the Adriatic continental lithosphere which represents,since
the middle Tertiary, the foreland domain of the peri-Adriatic
orogens. The Ionian Sea lies on a similarly undeformed
oceanic-like lithosphere [Finetti, 1982; Boccaletti et al.,
1984]. The Adriatic lithosphere is referred to by several
authors as a microplate, with a poorly defined southern
boundaryin the Ionian area [e.g., Lort, 1971; Vandenbergand
Zijderveld, 1982; Morelli, 1984; Anderson and Jackson,
1987]. In the present study no major discontinuities are
imaged in the velocity structurebetweenthe Adriatic and the
_._
12,536
MELEETAL.:PnVELOCITY
ANDANISOTROPY
IN ITALY
10' E
20 ø
50øN
50 ø
40*
40 ø
_
.-
0ø
22 ø
Direction
/
I
45 ø
77 ø
I
90 ø
Error
10 ø
20 ø
10øE
20øE
50øN
50 ø N
40* N
40 ø N
Anisotropy Error (km/s)
0ø
10øE
20øE
Figure 5b. Errors in the direction and amount of anisotropy.
the present-daytectonic regime of this chain. The internal uppermost mantle. Delamination of the Adriatic continental
the northernApenninesto the west
Apennineunits are characterizedby extensionaldeformation lithosphereunderthrusting
to rise at shallowdepth,
superimposed
onto compressional
featuresof post-Tortonian might have causedthe asthenosphere
age, and by high heat flow. Magmatismis found in Tuscany as already proposed by Reutter et al. [1990], $erri et al.
and northernTyrrhenianSea and geothermalfields are present [1993], and Mele et al. [1997]. The continentallithosphereis
in southernTuscany.The low-Q zone beneaththe Apennines not expectedto be involved in subductionprocesses[e.g.,
hasbeeninterpretedby Mele et al. [1996, 1997] as due to the Molnar and Gray, 1979]. However,its lowermostpart (the
rising of anomalouslyhot material(asthenosphere)
below the mantle lid) is colder and denser than the underlying
Moho discontinuity. The low Pn velocities imaged in the asthenospheric mantle, and detachment of the lower
presentstudylend further supportto an anomalously"hot" :lithospherefrom a underthrusting
plate may occurin
MELE ET AL.: Pn VELOC1TY AND ANISOTROPY IN ITALY
20 ø
10øE
50 ø N
12,537
50 ø
+
40 ø
40 ø
-1.5
-1.0
-.5
.5
1.0
1.5
Station Delays (s)
0ø
10ø
20 ø
Figure 6a. Stationstaticdelaysin Italy andsurrounding
regions.Early delays(circles)indicatethin crustor
highervelocitycrust.Latedelays(squares)
indicatethickcrustor slowervelocitycrust.
continental collision zones. In a delamination model [e.g.,
zones resulting from previousstudies[Mele et al., 1996,
Housemanet al., 1981], the subcrustallithospheredetaches 1997] and the mantle earthquake occurrence [lstituto
from the overlying crust and hot asthenospheric
material Nazionale di Geofisica (ING), 1997]. The main velocity
replacesit at shallow depth. Low velocitiesand high anomaliesimaged at mantle depth by previoustomography
attenuationof seismicwavesare expected[e.g., Seberet al., studies [Babuska and Plomerovd, 1990; Spakman, 1991;
1996]. Crustal extensionand collapsein orogenicbelts may Cimini and Amato, 1993; Spakmanet al., 1993] are reported
on a crosssectionperpendicular
to the regionaltectonictrend
alsobe a consequence
of thisprocess[e.g.,Dewey, 1988].
Figure9a showsthe low-velocityanomalies
resultingfrom of the northernApennines(Figure 9b). Beneaththe northern
the presentstudytogetherwith the Pn andSn attenuation Apenninessubductionis thought not to be active since
10øE
20 ø
50 ø
50 ø N
-+
++
+
40 ø
40 ø
0.0
0.1
0.2
0.3
Station Error (s)
o
10 ø
20 ø
Figure 6b. Averagestandarddeviationsfor the stationdelays.
12,538
MELE ET AL.: Pn VELOCITY AND ANISOTROPYIN ITALY
10 ø E
20 ø
50 ø N
50 ø
40 ø
40 ø
-0.3
-0.2
-0.1
0.0
+0.1
+0.2
+0.3
Pn Velocity 8.2 (km/s)
0 øFigure
7a.
Checkerboard 10 ø20
test model
with
cell size ofø 2 ø by 2 ø.
Tortonian, and present-daycompressionalong its outermost velocity anomalyimagedbeneatheasternSicily and Calabria
arc is due to the thrust of the chain over the Adriatic foreland
(Plate 1) can be related to this process. Beneath the
[Elter et al., 1975; Scandone and Patacca, 1984; Castellarin
Tyrrhenian basin, we find normal Pn velocity which is not
and Vai, 1986]. The occurrenceof earthquakesin the upper 90 expectedon the basisof the present-daytectonicsand the very
km
could
thus
be
related
to
the
deformation
of
the
underthrustingAdriatic lithosphere,while the high velocities
imaged at greater depth (Figure 9b) would represent the
subductedlithosphere.
In contrast,a well-defined Benioff zone as deep as 600 km
in the south Tyrrhenian Sea indicates that lithosphere
subduction
occurs
beneath
the
Calabrian
Arc.
The
low-
10ø
high Sn attenuationobservedin the area (see Figure 8).
However,the thin coverageof ray pathsover the Tyrrhenian
Sea may accountfor this discrepancy.
A close correlationbetweenthe regionaltectonictrend and
surface geology and the direction of anisotropicfast P n
velocity is observedalong the Italian peninsula.Pn velocity
along the northern Apennines and the Calabrian Arc is as
20'
50 ø N
50 ø
40 ø
40 ø
-0.6
-0.4-0.2
0.2
0.4
0.6
Station Delays (s)
0ø
10 ø
20 ø
Figure 7b. Stationdelayscalculatedfrom the checkerboard
testmodelshownin Figure7a.
MELEET AL.:PnVELOCITYAND ANISOTROPY
IN ITALY
•
z
o
•
12,539
o
12,540
MELE El' AL.: Pn VELOCITY AND ANISOTROPY IN ITALY
(a)
10 ø
(b)
15øE
extension
45 ø
compression
2000 rn
Apennines
X
Tvrrhenian
•
Sea
1000
Adriatic
Sea
.,,
, • "/...?_::d/X
....... ,:'....•,.:.•._',._x',.x',.,
...' ..",,
,..:,..:
•.•
X' 0
i..'"
/ / /
-100
,..:z•z.z.z.z.•.
.....
........'
,•.
40 ø N
•, ....'....'.•,.,.,.,.,...,..q
/'q•
-200
•Sy.v9.3-'
-300
....
•'•'• 7.9km/s
(this
study)
8.0km/s
(this
study)
• Pnattenuation
zone
[Mele
etal.,1996]
•:1•
ß Snattenuation
zone
[Mele
etal.,1997]
0
Distance
(km)
300
low-velocity
anomalies
high-velocity
anomalies
Figure 9. (a) Map of Italy showingthe low-velocityzones(this study),togetherwith the high-attenuation
zonesinferredin the uppermost
mantleby Mele et al. [1996, 1997]andwith the 1985-1995mantleearthquake
occurrence(squares)beneaththe northernApennines.The maximum magnitudeof the eventsis 4.0. The
projectionzone of the crosssectionXX' is also shown.(b) Cross sectionperpendicularto the northern
Apenninesshowingthe mantleearthquakes
occurrence
within the projectionareaalongwith the extentof the
uppermantlevelocityanomaliesresultingfrom the presentstudy(MRSHB98) andfrom previoustomography
studies(BP90 [Babuskaand Plomerov&1990],CA93 [Cimini and Amato,1993], andSvv93[Spakman,1991;
Spakmanet al., 1993]). The present-daytectonicregime is indicated.
much as 5% higher than Pn velocity in the perpendicular
direction,i.e., the directionof orogeniccompression
(Figure
5a). The preferredorientationof anisotropicmineralsin the
uppermantleis primarilycausedby late orogenicdeformation
of the original fabric of the rocks. The slow axis of olivine (b
axis) is expectedto align with the directionof compression,
while the fast axis (a axis) is perpendicularto the b axis. This
would cause an arc-parallel anisotropy, if the direction of
compressionis perpendicularto the arc or if the mantle flows
along strike of the subductedslab. Arc parallelanisotropyhas
been observedbeneaththe west coast of South Amertca [Russo
and Silver, 1994], the Aleutian arc [Yang and Fischer, 1995],
and Kamchatka[Fischer and Yang, 1994]. Regardlessof cause,
the correlation between the Pn structure and the regional
tectonicfeaturesin Italy suggeststhat a substantialportionof
uppermost mantle has been deformed by the collisional
processesthat originated the northern Apennines and the
Calabrian Arc. The fast polarization directionsof the SKS
waves inferred across the northern Apennine chain
[Margheriti et al., 1996] are consistentwith the direction of
Pn anisotropybeneaththe external units of the chain (Figure
5a). Toward the Tyrrhenian Sea both SKS and Pn show
rotating directionsof anisotropy,but they are rather different.
It is important to remind that whereasthe SKS waves sample
anisotropy vertically integrated within several hundred
kilometers of depth, the Pn waves sample the velocity
structure in the uppermost mantle just below the Moho
discontinuity.
For this reason,the comparison
betweenSKS
andPn anisotropy,
andin turnwith surfacegeology,simply
suggests
that anisotropyis ratherstablewith depthbeneath
the external Apennine arc, where the removal of mantle
lithosphere by delamination has not been relevant, but
significantly
variesat depthbeneath
theTyrrhenian
marginof
the peninsula.
A low-velocity anomaly lies beneath northern Albania
(Plate 1), wherethe Scutari-Pec
(S-P) shearzoneseparates
fromnorthto souththeDinarides
fromtheHellenides
(Figure
1). The trendof thesemountain
beltschanges
about30øacross
this tectonic line [e.g., Auboin and Ndonjaj, 1964].
Furthermore,
the directions
of Pn anisotropy
areuniformfrom
the Calabrian Arc to Albania, while very little or no
anisotropyis observednorthof the Albanianborder(Figure
5a). Paleomagneticinvestigationsshow that the S-P zone
acted as a major discontinuityin the Cenozoicevolutionof
the easternperi-Adriatic region. In fact, no significant
rotationis foundin the Dinaridesat leastfrom earlyEocene,
but a post-Eocene clockwise rotation of 45 ø affected the
externalHellenides.The basementmay be involvedin this
rotation, which implies that the S-P tectonic zone extendsat
leastto the deepcrust[Kisselet al., 1995; Speranzaet al.,
1995]. Our tomographyresults strongly suggestthat
deformation along the Scutari-Pec zone extends into the
uppermost mantle as well.
Partof thePannonian
basin,eastof Dinarides,
is imagedin
MELE ET AL.: Pn VEI_DCITY AND ANISOTROPY IN ITALY
the present study. This basin is a wide continental area of
Neogene-Quaternary subsidence and high heat flow which
formed behind the concaveside of the CarpathianArc (Figure
1). It is interpretedas a "Mediterranean-type"back arc basin
formed on a 25 km thick continentalcrust [e.g., Horvath and
Berckhemer, 1982]. For this area Posgay [1975] reports an
uppermost mantle P wave velocity of 8.2 km/s, which
decreasesto 7.7-7.8 km/s at about 60 km of depth. In the
presentstudywe find that velocitiesas low as 7.9 km/s occur
also at Moho depths,at least beneaththe westernpart of the
basin (Plate 1). Low Pn velocity is as expectedin a region of
lithospheric extension.
Finally, the low Pn velocities imaged beneath the western
Alps may be the result of drastic and abrupt changesof the
Moho depth, as interpreted also by Parolai et al. [1997].
Moho depth is in fact reported to double from 25 to 55 km
within
a few tens of kilometers
from
SE to NW
across the
12,541
Arc. Lateral heterogeneitiesin the uppermost mantle are
inferred from both the velocity and anisotropy fields in
northern Albania beneath the Scutari-Pec shear zone, which is
interpretedas the surfaceexpressionof a major lithospheric
discontinuity. Low seismic velocities at subcrustal depth
outline the lithospheric extension site of the Pannonian
basin.
Acknowledgments. We thankDaniel McNamaraand Nano Seeber
for providingconstructivecommentsand suggestions.
This work was
supportedby internalresearchfundsat IstitutoNazionaledi Geofisica
and by National ScienceFoundationgrantsEAR-9205257 and EAR9627855 at CornellUniversity.INSTOC contribution240.
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(ReceivedFebruary18, 1997; revisedJanuary28, 1998;
acceptedFebruary5, 1998.)